1. Field of the Invention
The invention relates generally to flat display panels. More specifically, the invention relates to a method and an apparatus for making glass sheet.
2. Background Art
There is a surge of interest in glasses having a high strain point in the display industry. These glasses are needed to make flat display panels for the next-generation liquid crystal displays (LCDs), e.g., active matrix LCDs (AMLCDs), and other advanced displays, e.g., plasma displays. Generally speaking, a strain point of at least 700° C. is desired. Preferably, the strain point is greater than 800° C. In the case of AMLCDs, the need for such a high strain point is dictated by the interest in bonding silicon chips or arrays directly onto glass substrates. Fabrication of poly-silicon on glass substrates is further facilitated by process temperatures of 900° C. or greater. In order to accomplish this objective, it is necessary for the thermal expansion behavior of the glass to be very similar to that of silicon, and for the strain point of the glass to be high enough so that compaction (also known as shrinkage or densification) and/or warping of the glass does not occur after the silicon chips are bonded to the glass and the glass is subsequently heated in further processing steps.
The two methods commonly used in manufacturing LCD substrates are the float process and the fusion process. Both of these processes require a refractory glass melter to deliver a stream of glass to a sheet-forming device. In the case of high strain-point glass compositions, a relatively large high-temperature glass melter is needed to deliver a high-quality stream of glass to the sheet-forming device. This is because high strain-point glasses have high fusion temperatures, typically in excess of 1700° C.
In the float process, a stream of molten glass is discharged from a melting furnace into a float furnace that contains a liquid metal medium. Typically, the metal is tin. The atmosphere in the float furnace is controlled to prevent oxidation of the tin. The molten glass floats and spreads out on the liquid tin in the form of a flat, continuous ribbon. The ribbon of glass is conveyed into an annealing lehr or cooling tunnel, where it is cooled at a controlled rate to ambient temperature. The cooled glass has a flat, smooth surface that requires a minimum of further finishing by processes such as grinding and polishing.
However, it is very difficult to form glasses having high strain points in an enclosure containing molten tin. This is because tin has high vapor pressures at temperatures in excess of 1050 to 1100° C. At the high forming temperatures required for high strain-point glasses, the molten tin will vaporize inside the float furnace and subsequently condense in colder parts of the furnace. In some cases, the condensation may be sufficiently high to create what is referred to as “tin rain,” a situation where tin rains on the glass and is incorporated on the glass surface.
In the fusion process, a glass-forming melt flows into a refractory trough and then overflows in a controlled manner from either side of the trough. A key advantage of this process is that the surface of the glass sheet, which is ultimately formed, does not come in contact with any refractory material or other forming equipment. Another benefit of the process is that it yields a very flat and uniformly thick sheet of glass. As a result, no secondary processing is needed to obtain a smooth, flat, and uniform sheet of glass for display applications. The fusion process requires glasses exhibiting a relatively high viscosity at the liquidus temperature. Typically, it is desirable to form the glass at viscosities in the range of 105 to 106 poise to obtain optimum flatness and uniform thickness.
A brief description of both the fusion draw and float processes are given in a manuscript entitled “Glass” by D. C. Boyd and D. A. Thompson, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, pp. 807-880 (see pages 860-863). The fusion draw process is also described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both issued to Dockerty. Unfortunately, neither the fusion draw process nor the float glass process is effective in producing flat sheet from a glass composition whose strain point exceeds 900° C.